1) what make is the HU
2) how old is it
3) go on-line, call the folk who make it, speak to them, try and get a very rough quote
4) check specs of something equivalent now

-compare between quote

+ postage costs

+ hassle
5) just buy a new one of costs don't stack up, cheap HU's do more than expensive ones used to

(check connections are the same)

.

.

.simples!